Sulfur-based cathodes are promising candidates for next-generation, high-energy batteries because of the high specific capacity (1675 mAh/g) and low cost of the active materials. However, lithium-sulfur battery technology faces a number of challenges that currently limits its widespread adoption. Among those challenges is the issue of polysulfide dissolution, which degrades battery performance through the loss of cathode active material and a rapid reduction in energy capacity.
In contrast to traditional lithium-ion based cathode materials, whereby lithium intercalates into a rigid host framework such as a layered oxide material (e.g., LiCoO2), lithium-sulfur based cathodes utilize a conversion mechanism for energy storage. In lithium-sulfur based cathodes a chemical reaction occurs between the lithium migrating from the anode and the sulfur in the cathode. One of the advantages of conversion electrodes is the potential for higher energy density as a result of being able to use most, if not all, of the available oxidation states of the sulfur. Another advantage is the elimination of a host framework that is electrochemically inactive.
There are two different potential initial states for a Li—S battery. In the first, the cathode initially contains Li2S, which corresponds to the discharged state. In the second, the cathode initially contains S, which corresponds to the charged state. Manufacturing a battery with its cathode initially in the discharged state has distinct advantages over manufacturing one in which the cathode begins in the charged state. In particular, one advantage is the lower cost of the cathode materials. Li2S is less expensive than pure Li and pure S. Another advantage is the use of superior anode materials such as high-capacity silicon anodes instead of lithium.
Furthermore, beginning with S in the cathode (charged state) typically requires the use of lithium metal anodes. While lithium is a promising anode material, the formation of dendrites on the anode during extended cycling limits the cycle life of the battery and imposes significant safety concerns.
Silicon based anodes can provide an alternative to lithium anodes, but a silicon based anode requires that the source of lithium be contained within the cathode. Present lithium-sulfur cathode materials do not meet this requirement. On the other hand, Li2S based cathodes allow for the use of high-capacity silicon anodes instead of lithium anodes.
Another major challenge for lithium-sulfur batteries is polysulfide dissolution: a process whereby reaction of the lithium with sulfur in the cathode produces a number of different lithium-sulfur compounds, such as Li2S8, Li2S6, Li2S4, Li2S2, Li2S. These reaction products are known as polysulfides and, with the exception of Li2S, are typically soluble in the electrolyte solutions of lithium batteries. The dissolution of polysulfides into the electrolyte solution allows for migration of these species to the anode and subsequent reaction with the anode, which forms a layer of Li2S on the surface and results in an irreversible loss of active sulfur material. This coating limits the usable capacity of the battery and contributes to a rapid capacity fade within the first several cycles. In fact, cycle-life is typically limited to less than 50 cycles for lithium-sulfur based cells.
These and other challenges can be addressed by certain embodiments of the invention described herein